The present invention concerns photographic cameras provided with state-of-focus or distance-measuring means, means viewable in the camera viewfinder for indicating the state of focus of the camera objective relative to the subject to be filmed, and means for manually adjusting the state of focus of the camera objective.
Cameras of the type in question are widely provided in motion-picture camera form and in still-camera form. The means viewable in the camera viewfinder for indicating the state of focus of the camera may, for example, be a calibrated scale with pointer, or a double-image range finder, or in the case of reflex cameras may comprise a matte viewing screen with an embossed or otherwise formed microprism array, or may be a range finder of the split- or cut-image type, or the like.
The indicating means in the viewfinder is typically coupled to the camera's manual focus adjuster, or else operates in dependence upon adjustments performed by the photographer using the focus adjuster. Thus, for example, when the user moves a manual control, most typically turns an adjuster ring, in order to change the state of focus of the camera objective, this serves first to focus the camera and simultaneously to change the state-of-focus indication presented to the user in the camera viewfinder.
These typical state-of-focus indicators are intended to make the user's focussing task easier to perform than if no state-of-focus indicator at all were provided and the user had to rely exclusively on his own subjective awareness of image sharpness. Thus, for example, the typical split- or cut-image range finder in a reflex camera allows the user to subjectively evaluate, directly, the degree of image sharpness, but additionally provides an auxiliary form of guidance, namely that the edges of the two halves of the split image be brought into register. However, the alertness and carefulness with which the user must look at and evaluate the degree of register of the edges of the two halves of the split image are fairly considerable, so that the user's task in performing a proper focussing operation may not be much facilitated relative to performing the focussing by observing the degree of image sharpness directly. Certainly, this is particularly true in the case of users having poor vision, or users whose vision makes it uncomfortable or difficult for them to observe a small and close image in a camera viewfinder.
Furthermore, state-of-focus indicators of the types in question provide a further difficulty and inconvenience, even for example in the case of viewers with acute vision who find it comfortable to observe the small and close image in the camera viewfinder. Specifically, even if the user can clearly see that the state of focus is greatly or slightly improper, the user, even if quite knowledgeable, does not in general know in which direction the state of focus should be adjusted. For example, if as the viewer peers through the viewfinder his fingers are at the same time resting on the focussing ring of the camera, the user does not in general have any idea in which direction he actually should turn the focussing ring. As a result, the user typically picks the direction arbitrarily, with a consequent 50-50 chance that he picks the direction that will not actually worsen the state-of-focus situation. Indeed, after the first successful one of such manual adjustments, when the careful user then attempts to improve the state of focus just a little bit more, it is common that he will, once again, be unaware of the direction in which he should move the focus adjuster, so that once again he must make a 50-50 arbitrary choice of direction, followed by another manual adjustment if his first choice was the unlucky one. This is actually quite annoying, especially because many users are very careful and repeatedly try to improve the state of focus an extra little bit or else, after a completely successful focussing operation, nevertheless proceed to repeat the focussing operation anew from lack of confidence.
Of course, cameras are presently known having completely automatic focussing means, such that the user is not called upon to participate at all in the focussing operation. Completely automatic focussing means are disclosed, by way of example, in German published allowed patent applications DT-AS 1,447,469 and DT-AS 2,455,407 and in German published patent application DT-OS 2,410,681. These automatic focussing means typically include means for generating a signal indicative of the sense of the state-of-focus error, an electric motor which adjusts the state of focus of the camera, and circuit means for energizing the adjusting motor in a direction causing it to adjust the state of focus in the appropriate sense, i.e., to automatically reduce the state-of-focus error. The negative-feedback control of the operation of the electrical adjusting motor typically involves the use of motor-control circuitry in addition to the means required merely to generate a signal indicating the sense or direction of the state-of-focus error.
The self-evident drawbacks of such completely automatic focussing systems include the cost of the adjusting motor and the circuitry for controlling motor energization, the space consumed by the adjusting motor and it weight, the power drain upon the camera's battery attributable to the adjusting motor, the possibility that the adjusting motor may malfunction and require repair, and so forth. In addition, the adjusting motor will typically be coupled to the focussing mechanism of the camera through a mechanical transmission, which further adds to cost, complexity, size, weight and the possible need for repair.